Two artefacts from the treasure of 6th Century BC Lydian ruler, King Croesus, have been stolen from a museum in Turkey and replaced with fakes.

For years they were part of a legal battle between Turkey and a US museum after they were stolen in the 1960s and sold to New York's Metropolitan Museum.

Nine people, including the director at the Usak Museum in western Turkey, have been arrested over this latest theft.

The king's legendary wealth gave rise to the phrase "as rich as Croesus".

The stolen objects - a coin and a golden brooch in the form of a winged horse-shaped sea monster, a hippocamp - were returned to Turkey in 1993 after the Metropolitan Museum admitted it had known the objects were stolen when they purchased them as part of a 363-piece treasure.

The treasure is known as the Lydian Hoard or as the Karun Treasure.

One object - a golden bird figure - was reportedly stolen during the return and the rest of the collection spent three years in Ankara before being sent to Usak.

Investigators were alerted to this latest theft when they received an anonymous letter in December.

They said in a report that the switch probably took place between March and August 2005 and that it could not have been carried out "without the knowledge of museum authorities," the Milliyet daily newspaper says.

On Monday, the governor of Usak, Kayhan Kavas, said that police had detained nine suspects, including Usak Museum's director, Kazim Akbiyikoglu, in connection with the theft.